With the recent developments in magnetic recording, the demand for high video quality and high audio quality is becoming stronger and stronger, and improvements in electromagnetic characteristics, especially by employing finer ferromagnetic particles or heightening the loading density of ferromagnetic particles are progressing. Another requirement for improved video and audio tapes is diminished noise around carriers, and heightened C/N ratio achieved by making magnetic tape surfaces smoother. In addition, because such smoother surfaces result in increased friction coefficients, further improvements in running properties and durability are simultaneously required. Further, magnetic recording media are also required to be manufactured at low cost, since such media are being consumed in large quantities. In order to meet these requirements, a technique has been developed for forming a multilayered magnetic coating. This technique is advantageous in that proper ferromagnetic particles can be used, so as to impart high video quality-attaining properties to the upper layer and to impart high audio quality-attaining properties to the lower layer. Magnetic recording media with such a multilayered magnetic layer have, at the same time, another advantage in that they can be produced at low cost, because materials suited for the individual layers can be employed. At present, known means for improving the electromagnetic characteristics of a magnetic layer are to improve the surface properties of the magnetic layer, and to heighten the loading density of ferromagnetic particles in the magnetic layer.
As an example of a method for heightening the loading density of ferromagnetic particles in a magnetic layer, use may be made of a method in which the proportions of solid ingredients, other than ferromagnetic particles, i.e., binder, lubricant, antistatic agent, abrasive, etc., are decreased; as another example of a method for heightening the loading density of ferromagnetic particles involves dispersing the ferromagnetic particles to a high degree by means of special dispersants or dispersing equipment. However, the former method is defective in that the magnetic tape thus produced has poor physical properties, particularly has the decreased adhesion strength and tensile strength of the coating; the resulting tape also has a high friction coefficient and a high surface electrical resistance, so that the magnetic tape has significantly impaired running durability. The latter method is defective in that the dispersing step requires considerable time, and in that properties originally possessed by the ferromagnetic particles are lost; in particular, the acicular shape of the particles is destroyed, resulting in a worsened noise level and other problems. Hence, each method fails as a sufficient solution to the above problems.
In an attempt to improve electromagnetic characteristics, such as, sensitivity, and specifically in order to obtain a magnetic recording medium which has high loading density of ferromagnetic particles, without impairing physical properties of the tape, and which also has high sensitivity and high density, a technique has been proposed in JP-A-58-200425 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") in which a lubricant, antistatic agent, and abrasive, additives which govern running durability, are incorporated into a second layer (upper layer), but are not incorporated into a first magnetic layer (lower layer), which does not particularly affect running durability. This magnetic recording medium gave good results with respect to S/N, head abrasion, and still durability, but has proved to be poor in running durability, because the edges of the first magnetic layer are prone to suffer damage when the tape is run with the edges sliding while undergoing friction.
On the other hand, JP-A-58-9215 discloses a magnetic recording medium comprising a surface layer containing no, or only a slight amount of, fine particles of an inorganic oxide, such as, alumina, and a lower magnetic layer containing fine particles of an inorganic oxide, such as, alumina, for the purpose of diminishing magnetic head damage or wear when the magnetic recording medium is run while in contact with the magnetic head and, further, for the purpose of improving the durability of the magnetic recording medium. According to JP-A-58-9215, the recording medium is free from the problem that, although the presence of alumina in a surface layer improves durability, the exposed alumina particles scratch or chip, or accelerate the wear, of magnetic heads. However, the above magnetic recording medium showed significantly impaired still durability, though it caused little head wear, because of the small proportion of abrasive in the second magnetic layer (upper layer).
Accordingly, for the purpose of attaining improvements in audio level, which is lowered by tape edge damage, and in still durability and S/N, which have been insufficiently achieved with the conventional techniques, the present applicant(s) proposed a magnetic recording medium comprising a second magnetic layer containing an abrasive having a Mohs' hardness of 8 or more in an amount of from 1 to 15 wt % based on the amount of ferromagnetic particles contained in the second magnetic layer, and a first magnetic layer containing an abrasive having a Mohs' hardness of less than 8 in an amount of from 5 to 50 wt % based on the amount of ferromagnetic particles contained in the first magnetic layer; the first magnetic layer also contained the abrasive in an amount larger than the amount of abrasive in the second magnetic layer (See JP-A-2-105327 corresponding to U.S. Pat. No. 5,080,967). Although this proposal proved to improve S/N and the lowering of audio level, and also to improve still durability at ordinary temperature, the proposed technique failed to attain sufficient improvements under severe conditions, such as improvements in still durability at low temperatures, e.g., -5.degree. C., edge damage in high-humidity environments, e.g., 25.degree. C./80% RH, and clogging in low-humidity environments, e.g., 25.degree. C./10% RH.
Under these circumstances, intensive studies were made of physical property combinations of nonmagnetic powders to be contained in a first magnetic layer and a second magnetic layer, amounts of such nonmagnetic particles to be used, kind of binder, etc. As a result, it has been found that the above-described drawbacks can be eliminated remarkably when nonmagnetic powders having different pH values are incorporated into a first magnetic layer and a second magnetic layer, respectively. The present invention has been completed based on this finding.